Re: [PATCH v3 seccomp 3/5] seccomp/cache: Lookup syscall allowlist for fast path

[Kees Cook]

On Wed, Sep 30, 2020 at 10:19:14AM -0500, YiFei Zhu wrote:
> From: YiFei Zhu <yifeifz2@xxxxxxxxxxxx>
> 
> The fast (common) path for seccomp should be that the filter permits
> the syscall to pass through, and failing seccomp is expected to be
> an exceptional case; it is not expected for userspace to call a
> denylisted syscall over and over.
> 
> This first finds the current allow bitmask by iterating through
> syscall_arches[] array and comparing it to the one in struct
> seccomp_data; this loop is expected to be unrolled. It then
> does a test_bit against the bitmask. If the bit is set, then
> there is no need to run the full filter; it returns
> SECCOMP_RET_ALLOW immediately.
> 
> Co-developed-by: Dimitrios Skarlatos <dskarlat@xxxxxxxxxx>
> Signed-off-by: Dimitrios Skarlatos <dskarlat@xxxxxxxxxx>
> Signed-off-by: YiFei Zhu <yifeifz2@xxxxxxxxxxxx>

I'd like the content/ordering of this and the emulator patch to be reorganized a bit.
I'd like to see the infrastructure of the cache added first (along with
the "always allow" test logic in this patch), with the emulator missing:
i.e. the patch is a logical no-op: no behavior changes because nothing
ever changes the cache bits, but all the operational logic, structure
changes, etc, is in place. Then the next patch would be replacing the
no-op with the emulator.

> ---
>  kernel/seccomp.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 52 insertions(+)
> 
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index f09c9e74ae05..bed3b2a7f6c8 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -172,6 +172,12 @@ struct seccomp_cache_filter_data { };
>  static inline void seccomp_cache_prepare(struct seccomp_filter *sfilter)
>  {
>  }
> +
> +static inline bool seccomp_cache_check(const struct seccomp_filter *sfilter,

bikeshedding: "cache check" doesn't tell me anything about what it's
actually checking for. How about calling this seccomp_is_constant_allow() or
something that reflects both the "bool" return ("is") and what that bool
means ("should always be allowed").

> +				       const struct seccomp_data *sd)
> +{
> +	return false;
> +}
>  #endif /* CONFIG_SECCOMP_CACHE_NR_ONLY */
>  
>  /**
> @@ -331,6 +337,49 @@ static int seccomp_check_filter(struct sock_filter *filter, unsigned int flen)
>  	return 0;
>  }
>  
> +#ifdef CONFIG_SECCOMP_CACHE_NR_ONLY
> +static bool seccomp_cache_check_bitmap(const void *bitmap, size_t bitmap_size,

Please also mark as "inline".

> +				       int syscall_nr)
> +{
> +	if (unlikely(syscall_nr < 0 || syscall_nr >= bitmap_size))
> +		return false;
> +	syscall_nr = array_index_nospec(syscall_nr, bitmap_size);
> +
> +	return test_bit(syscall_nr, bitmap);
> +}
> +
> +/**
> + * seccomp_cache_check - lookup seccomp cache
> + * @sfilter: The seccomp filter
> + * @sd: The seccomp data to lookup the cache with
> + *
> + * Returns true if the seccomp_data is cached and allowed.
> + */
> +static bool seccomp_cache_check(const struct seccomp_filter *sfilter,

inline too.

> +				const struct seccomp_data *sd)
> +{
> +	int syscall_nr = sd->nr;
> +	const struct seccomp_cache_filter_data *cache = &sfilter->cache;
> +
> +#ifdef SECCOMP_ARCH_DEFAULT
> +	if (likely(sd->arch == SECCOMP_ARCH_DEFAULT))
> +		return seccomp_cache_check_bitmap(cache->syscall_allow_default,
> +						  SECCOMP_ARCH_DEFAULT_NR,
> +						  syscall_nr);
> +#endif /* SECCOMP_ARCH_DEFAULT */
> +
> +#ifdef SECCOMP_ARCH_COMPAT
> +	if (likely(sd->arch == SECCOMP_ARCH_COMPAT))
> +		return seccomp_cache_check_bitmap(cache->syscall_allow_compat,
> +						  SECCOMP_ARCH_COMPAT_NR,
> +						  syscall_nr);
> +#endif /* SECCOMP_ARCH_COMPAT */
> +
> +	WARN_ON_ONCE(true);
> +	return false;
> +}
> +#endif /* CONFIG_SECCOMP_CACHE_NR_ONLY */
> +
>  /**
>   * seccomp_run_filters - evaluates all seccomp filters against @sd
>   * @sd: optional seccomp data to be passed to filters
> @@ -353,6 +402,9 @@ static u32 seccomp_run_filters(const struct seccomp_data *sd,
>  	if (WARN_ON(f == NULL))
>  		return SECCOMP_RET_KILL_PROCESS;
>  
> +	if (seccomp_cache_check(f, sd))
> +		return SECCOMP_RET_ALLOW;
> +
>  	/*
>  	 * All filters in the list are evaluated and the lowest BPF return
>  	 * value always takes priority (ignoring the DATA).
> -- 
> 2.28.0
> 

Otherwise, yup, looks good.

-- 
Kees Cook